The present invention relates to a radio base station device constituting a radio communication system.
Orthogonal Frequency-Division Multiple Access (OFDMA) is a user multiplexing scheme that is often used in radio communications. In OFDMA, simultaneous access of a plurality of mobile terminals (MTs) is realized by assigning some of a large number of subcarriers, which are prepared by the Orthogonal Frequency Division Multiplexing (OFDM) scheme, to the mobile terminals as frequency resources. The OFDMA scheme requires assignment of frequency resources used in data communication before data transmission. In a cellular radio system using the OFDMA scheme, a base station (BS) determines the frequency resource assignment, and notifies each mobile terminal of the frequency resource assignment information through a dedicated control information channel.
In the downlink data transmission from the base station to the mobile terminals, first, the base station assigns frequency resources to each mobile terminal according to the amount of data to be transmitted to the particular mobile terminal, and the like. Each mobile terminal is notified by the base station of the frequency resource assignment information through the control information channel, at the same time as the data transmission or prior to the data transmission. The base station transmits data by using the frequency resources assigned to each mobile terminal. The mobile terminal receiving the data from the base station, determines which frequency resources are used for the data transmission, according to the frequency resource assignment information from the base station. Then, the particular mobile terminal receives the data based on the frequency resources determined as described above.
Further, in the uplink data transmission from the mobile terminals to the base station, first, each mobile terminal notifies the base station of a data transmission request and information on the desired data amount to be transmitted. The base station assigns frequency resources to each mobile terminal based on the notification, such as the data transmission request, from the particular mobile terminal. Each mobile terminal is notified by the base station of the frequency resource assignment information through the control information channel. Then, each mobile terminal determines which frequency resources should be used for the data transmission, and transmits data based on the frequency resources determined as described above. The base station receives the data by using the frequency resources assigned to each mobile terminal.
As described above, OFDMA can realize data communication with adaptive bandwidth assignment according to the amount of data to be transmitted, by sharing the information on the frequency resource assignment determined by the base station to the individual mobile terminals, between the base station and the individual mobile terminals.
In the cellular radio system using OFDMA, different frequency resources are assigned based on the mechanism described above. Thus, in general, there is no interference between the mobile terminals that communicate with the same base station. Rather, inter-cell interference is dominant in the environment in which the same frequency resource is assigned to mobile terminals that communicate with a plurality of neighboring base stations, respectively. For this reason, the OFDMA system requires a mechanism for controlling the inter-cell interference.
Fractional Frequency Reuse (FFR) is being studied as an inter-cell interference control scheme in the OFDMA system. In FFR, the inter-cell interference is reduced by dividing the frequency band into a plurality of subbands, and by using different subbands between neighboring base stations or changing the transmission power assignment pattern for each subband between neighboring base stations. The implementation of the FFR technology is described in JP-A No. 510967/2009 and in International Publication No. WO 08/004,299.
In the standardization body 3GPP, radio communication systems using OFDMA and Discrete Fourier Transform-Spread (DFT-S)-OFDMA are standardized as Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN). Further, 3GPP TS 36.423 V9.1.0, 8.3.1 Load Indication provides an inter-base station (BS) interface X2 to support the inter-cell interference control by FFR, and the like, in collaboration with the neighboring base stations. The inter-BS interface X2 exchanges information about the transmission power, and the like, between base stations.
In the inter-BS interface X2, downlink transmission power information called Relative Narrowband Transmit Power Indication (RNTP) is exchanged between base stations in the smallest unit of frequency resource assignment called a resource block (RB). Each base station learns in which frequency the transmission power of the neighboring base station is large, by using RNTP notified by the neighboring base station. In the frequency in which the transmission power of the neighboring base station is large, in general, the received interference power of the mobile terminal that communicates with the own base station is large. Further, in general, the mobile terminal that is located close to the cell edge is closer to the neighboring base station than the mobile terminal that is located close to the cell center. Thus, the downlink interference power tends to be higher in the mobile terminal that is located close to the cell edge.
Further, in the inter-BS interface X2, the interference information received by a base station in the uplink is exchanged between base stations as an interference overload indication (OI). OI contains the received interference power information of the base station in each RB. Furthermore, in the inter-BS interface X2, the information about sensitivity to the uplink interference is exchanged between base stations as a high interference indication (HII). HII contains information about RB which is not desired to be used for cell-edge mobile terminals in nearby cells. In general, a mobile terminal that is located in the cell edge of a base station, and a mobile terminal that is located in the cell edge of the neighboring base station, can be highly interactive interference sources.
In E-UTRA and E-UTRAN, the communication interface between base stations called the inter-BS interface X2 is used to exchange subcarrier assignment information in each base station as well as transmission power information.
However, the inter-BS interface X2 is not necessarily supported between all installed base stations. For example, in E-UTRA and E-UTRAN, a base station with a relatively small communication area, which is called a femto-cell (Home eNB), does not support the inter-BS interface X2. In such a case, the inter-cell interference control is not likely to be provided in collaboration between base stations. As a result, the throughput is reduced due to inter-cell interference.
When a new femto-cell BS is installed within the communication area in which a macro-cell BS has been installed for improving the area quality, etc., it is desirable that the presence of the femto-cell BS has no influence on the communication quality in the existing macro-cell BS that has been installed based on the communication area design. However, due to interference given by the newly installed femto-cell BS, the state of the interference of the existing macro-cell BS is changed. As a result, the communication quality is unstable in the existing macro-cell BS. Or, the communication quality is unstable in the newly installed femto-cell BS due to the interference from the existing macro-cell BS.